The third-generation mobile communication LTE (Long Term Evolution) system is composed of an E-UTRAN (Evolved Universal Terrestrial Radio Access Network), a UE (User Equipment), and an EPC (Evolved Packet Core). Wherein, the E-UTRAN is composed of Enhanced node B (eNB), and may be also called as an eNB; the UE may be also called as a terminal; the EPC includes a Mobility Management Entity (MME), a Packet Data Gateway (P-GW), and a Service Gateway (S-GW).
In a LTE system, in order to save the battery/power consumption of the UE, the eNB may configure a DRX function for the UE through the Radio Resource Control (RRC) protocol so as to control the activity or act of monitoring the a PDCCH (Physical Downlink Control Channel) of the UE. In a state of radio resource control connection, if DRX is configured, the UE is allowed to monitor the PDCCH discontinuously; otherwise, the UE monitors the PDCCH continuously. The RRC configures timers and related parameters required by the DRX operation, and the timers and related parameters include a duration timer; a DRX inactivity timer; a DRX retransmission timer, which is configured for each downlink HARQ (Hybrid Automatic Repeat Request) procedure except the downlink HARQ procedure specified for receiving the Broadcast Control Channel; long DRX cycle; a DRX initial offset value. Optionally, there are also a DRX short cycle timer and short DRX cycle. Each downlink HARQ procedure, except the downlink HARQ procedure specified for receiving the Broadcast Control Channel, is further configured with a HARQ Round-trip Time (RTT) timer.
In order to describe the DRX act of the UE, a concept of PDCCH sub-frame is introduced. For a UE working in a FDD (Frequency Divided Duplex) mode, the PDCCH sub-frame can represent any sub-frame; for a UE working in a TDD (Time Divided Duplex) mode, the PDCCH sub-frame only refers to downlink sub-frames and special sub-frames containing DwPTS.
Activity time refers to the time when the UE monitors the PDCCH in the PDCCH sub-frame.
Media access control contention resolution timer refers to the number of continuous PDCCH sub-frames of the PDCCH that the UE should monitor after the UE transmits a message 3 (Msg3) of a random access process.
DRX cycle means that periodic repetition of the duration accompanied by a period of possible inactivity time.
DRX inactivity timer defines the number of continuous PDCCH sub-frames after the UE successfully decodes the PDCCH, indicating that there is initial uplink or downlink user data transmission of the UE.
DRX retransmission timer defines the maximum number of continuous PDCCH sub-frames when the UE initially expect for the downlink retransmission.
DRX short cycle timer defines the number of continuous sub-frames when the UE conforms to short DRX cycle.
HARQ RTT timer defines the minimum number of sub-frames before the downlink HARQ retransmission expected by the UE.
Duration timer defines the number of continuous PDCCH sub-frames at the beginning of DRX cycle.
FIG. 1 illustrates the basic working principle of DRX in the existing LTE system: when DRX cycle is configured, the activity time of the UE includes the time when the duration timer, or DRX inactivity timer, or DRX retransmission timer, or media access control contention resolution timer operates; or the time when the SR (Scheduling Request) transmitted on the physical uplink control channel (PUCCH) is pending, i.e., the time of waiting for uplink grant (UL Grant) when the UE transmits the SR; or the time of possible occurrence for assigning UL Grant to the pending HARQ retransmission when there are data in the corresponding HARQ buffer area; or after successfully receiving a random access response message for explicitly transmitted random access preamble, the UE does not receive the indication of the PDCCH that there is new transmission for the cell radio network temporary identifier (C-RNTI) of the UE.
When DRX is configured, the UE executes the following operations in each sub-frame.
If short DRX cycle is used and [(SFN*10)+subframe number] modulo (shortDRX-Cycle)=(drxStartOffset) modulo (shortDRX-Cycle) is met; or if long DRX cycle is used and [(SFN*10)+subframe number] modulo (longDRX-Cycle)=drxStartOffset is met, a duration timer is started in the corresponding sub-frame. For a TDD mode, the duration timer may be started in an uplink sub-frame. Wherein, SFN is the number of the system frames of the recorded sub-frames; subframe number is the number of sub-frames; shortDRX-Cycle is short DRX cycle; drxStartOffset is the initial offset of DRX; longDRX-Cycle is long DRX cycle.
If time is out in the sub-frame and the data in the corresponding HARQ procedure soft buffer area are not successfully decoded, a DRX retransmission timer is started for the corresponding HARQ procedure.
If a DRX command media access control element (DRX command MAC CE) is received, the duration timer is stopped; the DRX inactivity timer is stopped.
If the DRX inactivity timer times out in the sub-frame or a DRX command MAC CE is received: if short DRX cycle is configured, the DRX short cycle timer is started or restarted, and short DRX cycle is used; otherwise long DRX cycle is used.
If the DRX short cycle timer times out in the sub-frame, long DRX cycle is used. During this period, for PDCCH sub-frames, except the sub-frames operated by half-duplex FDD UE and sub-frames overlapping with measurement gap, the UE needs to execute the following operations:
monitoring PDCCH;
if the PDCCH indicates downlink transmission or there is preconfigured downlink assignment in the sub-frame, starting the HARQ RTT timer for the corresponding HARQ procedure; stopping the DRX retransmission timer for the corresponding HARQ procedure;
if the PDCCH indicates new downlink or uplink transmission, starting or restarting the DRX inactivity timer;
the UE receiving or transmitting a HARQ feedback when needing to receive or transmit the HARQ feedback no matter whether the UE is monitoring the PDCCH or not.
In order to meet the increasing requirement for large-bandwidth and high-speed mobile access, the 3GPP (Third Generation Partnership Projects) puts forwards the LTE-A (Long-Term Evolution advanced) standards. The LTE-A adopts a series of new technologies to expand the frequency domain and spatial domain based on LTE so as to achieve the purpose of improving frequency spectrum utilization ratio and increasing system capacity. Among them, carrier aggregation technology can aggregate two or more component carriers to support wider frequency band, for example, reaching 100 MHz and frequency spectrum aggregation. The UE may receive/transmit data on one or more component carriers simultaneously, and may configure to aggregate component carriers of different numbers and different bandwidths in uplink/downlink. Each component carrier corresponds to one HARQ entity and transmission channel. On each downlink component carrier there is a separate PDCCH for indicating the resource assignment on the present component carrier and resource assignment on other component carriers, there are two cases, downlink resource assignment and uplink grant, and the PDCCH has 0-3 bit for indicating component carrier identifier information. For an extended component carrier, there may be no PDCCH, and its resource assignment is indicated by other component carriers having a PDCCH. In LTE-A carrier aggregation, there may be one component carrier as the serving cell of the UE for providing functions such as synchronization, access, system information and paging; or a plurality of component carriers are used as the serving cells of the UE for providing the above functions, wherein, one serving cell may be configured to be a primary serving cell, which serves as the preferred or primary cell during measurement reference, switching, and random access. In a plurality of component carriers, backward compatibility component carriers, non-backward compatibility component carriers or extended component carriers may be configured. Wherein, the backward compatibility component carriers can provide UE access of a LTE system and a LTE-A system, and provide system messages, synchronization channels, random access resources, physical downlink control channels (PDCCH) and so on. The non-backward compatibility component carriers can provide UE access of a LTE-A system, and provide system messages, synchronization channels, random access resources and so on, and the physical downlink control channel may be configured, or may not be configured but downlink resource assignment or uplink grant is implemented using the PDCCHs of other component carriers. The above two types of component carriers may exist standalone. Extended component carriers, generally existing as resources, may provide some enhancing or extending functions, adopt different control signaling formats, and may provide or may not provide system information, and may be configured or may not be configured with a physical downlink control channel.
Among DRX solutions of carrier aggregation in a LTE-A system, currently there are three basis solutions: 1) consistent DRX for component carriers: all component carriers use a set of DRX configurations, conform to the same DRX act; 2) separate DRX for component carriers: each component carrier is configured with DRX respectively, and its DRX act is independent from others; 3) DRX based on a anchor component carrier: DRX is configured on the anchor component carrier, and DRX configuration or act of other non-anchor component carriers is partially or fully subject to the anchor component carrier. In the above three solutions, the DRX solution based on anchor component carrier and other two DRX solutions are still in conception stage currently, and no specific perfect solution is clarified yet.